The present invention relates to a method for making refractory bodies; and more particularly, it pertains to a method for hot forging of ceramic refractory blocks. The materials with which the invention is concerned include all inorganic non-metallic ceramic materials which are characterized as having low thermal conductivity, and which exhibit fluid-like properties at high temperature. That is, at temperatures about 1100.degree. depending on the material, some of the material is a highly viscous liquid and the remainder is solid. Such materials are referred to herein as "pyroplastic", and this term is not intended to exclude materials which are, in a strict sense, thermo-viscous. Examples include clays, feldspars, porcelain bodies and other silicates; periclase-Al.sub.2 O.sub.3, periclase-iron, etc. Blocks of these materials are useful, for example, in the steel industry in the bed of a steel-making furnace.
Ceramic refractory materials are used in industrial furnaces because they have a high melting point and because they resist corrosive chemical attack by the liquid metals used in processing and by the slags that are present during processing. However, the strong chemical bonding which brings about the high melting point of a ceramic refractory material also makes them good electrical insulators, and they have the characteristic of great strength. Thus, the present invention has uses beyond the indicated preferred usage.
The manufacture of refractories usually begins with finely powdered material, either natural clays or chemical compounds, which are formed and then fired to produce the final product. Some applications of refractories, particularly in the steel industry, require resistance to corrosion by processing liquids. For this purpose, it is desirable that the refractories be completely pore free. Because of the nature of refractories, pores are present, and they may be thought of as two separate kinds. Some pores are closed or reticulated, while others are open and thus capable of being saturated from the outside with liquids they contact. Both kinds of pores contribute to the total porosity of a ceramic body and make it easier for corrosive liquid to penetrate and dissolve the refractory. Further, pores substantially reduce the load-bearing capability of a ceramic body.
The strength of ceramic materials is known to improve dramatically when the last few percent of pore volume is reduced or removed. Further, the load-bearing capacity and corrosion resistance of refractories at elevated temperatures (such as in steel-making furnaces) are often greatly improved by eliminating porosity.
Refractory ceramics may be made by processes wherein raw plastic material such as clays or grain-sized aggregates such as magnesia refractories are formed into a suitable shape and fired in a kiln. During firing, the volume shrinks. After removal from the kiln, the bodies are cooled to room temperature. In this process, the final finished product is invariably porous. The technology of raw material preparation, forming and firing, is highly developed but the porosity always remains in the order of 15-25 percent of the exterior volume of the piece.
Refractory blocks may also be formed by the sintering process which requires grinding the materials to produce very fine granules and firing to extremely high temperatures. Porosity of the finished block may be reduced to two to five percent, but there is a substantial shrinkage in the volume of the block. This process is expensive and there is poor dimensional control of the finished product.
In metallurgy, the term "hot forging" has been applied to the plastic deformation of a preheated metal blank, which is transferred from the preheat furnace, struck with one die into a second die, the dies being unheated and at a much lower temperature than the piece of metal being forged. Such hot forging of metals has been done with drop hammers, steam hammers, air rams, or quick-acting hydraulic rams. Sometimes multiple impacts are employed. The contact time for each impact is relatively short. Such "hot forging", as far as is known, has not been employed commercially for ceramic refractory materials.
High density ceramic refractorys are commercially manufactured by a method known as "hot pressing". In this method, the refractory bodies or blocks are fired to the required temperature in an oven and then pressure is applied while the blocks are still in the oven, so that the pressing dies are also hot. After the application of pressure, the blocks are permitted to cool to room temperature in a controlled environment.
The material most often used for the molds in hot pressing ceramic refractories is graphite because of its high hot strength and its ability to withstand the intense heat of the furnace. However, the graphite dies or molds wear out quickly because of the poor abrasion resistance of graphite. These molds also oxidize, thus resulting in destruction of the molds and a loss in the ability to control the size of the ceramic body during pressing. The wear problem is caused principally because the dies are brought into direct contact with the ceramic refractory material which is highly abrasive. Thus, the present commercial hot pressing process is expensive and produces high cost, specialized materials.
Some prior patents have suggested procedures for hot forging ceramic bodies (U.S. Pat. Nos. 1,809,214 and 1,809,215), including employing cool molds or rollers. In the preferred process of the cited patents, as described therein, the ceramic body is subjected to a series of rolling operations. Further, the references indicate that heated molds or rollers are preferably employed. The reason given is: "The molds might possibly be cold but are preferably hot, in order not to extract too much heat from the mass of material during the early stages of the molding operation." As far as is known, the procedures taught by the Pine et al patents have not been employed commercially, and there is no established process today for the hot forging of ceramic materials as distinguished from hot pressing procedures, which are carried out within ovens, and are subject to the disadvantages and limitations described above. Some improvements in hot pressing apparatus and procedures have been described. (See, for example, U.S. Pat. No. 3,303,533.) In the process and apparatus of the cited patent, silicon carbide rams are employed within the furnace for the molding operation rather than graphite, since it is stated that silicon carbide reduces heat loss from the ceramic body being compressed. Thus, the general teaching of the prior art with respect to the compression of ceramic bodies in a pyroplastic condition is to minimize heat loss, either by conducting the compression within a furnace, or by heating the mold or rolls which perform the compression.
In hot pressing, both wear and temperature contribute to the deterioration of the die surfaces. In a process of molding an abrasive powder, two different types of abrasion can be considered. The first occurs during the initial compression of the powder, and this involves movement of powder particles relative to the die walls as compression occurs. The greatest relative movement between the powder and the die walls occurs in the first stages of compression when the forces are relatively small. Thus, this type of wear is not severe.
The second type of wear occurs when the forging is removed from the die cavity. Normally, the side walls of the die cavity are formed as a rigid unit, and upper and lower die surfaces (or one of them) are pressed relative to each other to induce the compression force. The body is removed from the mold after the pressure is released by removing the top surface and then forcing the body from the rigid side surfaces by forcing the bottom die piece upwardly. Abrasion against the side surfaces is severe because the forging has become rigid and cooled somewhat. The side particles which had been forced tightly against the die side walls during compression are then translated parallel to the plane of the die side walls thus causing severe abrasion.
In hot forging, it appears that the movement of the forging from the die or mold is also a major source of die wear. Hence, the present invention has as one objective, the provision of a commercially feasible method of forging hot ceramic bodies wherein the wear experienced by the die surfaces is minimized, and the operating temperature of the die surfaces are greatly reduced.
In the method of this invention, ceramic bodies of the general shape in which they finally are desired can be delivered from a furnace to a roller conveyor. The bodies are porous and at a pyroplastic temperature. A lubricant may be applied to the surface of the block or to the surfaces of the mold. The block is picked up by means of a clamp and delivered to a hydraulic press. The press includes a mold comprised of a plurality of individual pieces, fitting together to form the desired shape and adapted to come together and break away rapidly. Each mold piece is provided with a hydraulic cylinder and piston rod unit, and all of the mold pieces act in combination to provide the mold for exerting pressure on the ceramic body, the interior of which is still incadescent. The pressure exerted on the body within the mold can be in the range of 2,000 to 10,000 psi; and it need be exerted for only a short period of time -- of the order of a few seconds (viz. 1 - 5 seconds). The mold can then be broken apart and the mold pieces removed from the block except for one which supports it, and the travelling clamp is actuated to transport the block onto a second roller conveyor which feeds the block into an annealing oven.
The walls of the mold are cooled, but because they break away from the compressed body in a direction perpendicular to the surface of the body, sliding friction between the ceramic body and the mold surfaces is greatly reduced. Sliding friction, as used herein refers to the type of friction that results from rubbing two surfaces together. The body or the mold surfaces may be coated with a lubricant to further reduce friction between the block and the mold surfaces.
Because of the contact of the cold inner surfaces of the mold with the hot ceramic body during the compression, surface cooling of the body is inevitable. Such surface cooling would be expected to reduce the temperature of the body to below the pyroplastic temperature range, which would cause the cooled outer portions of the body to solidify without appreciable consolidation, the outer surface portions remaining porous and of relatively low density without strength increase. However, by the method of this invention, the compression can be carried out so rapidly that a high temperature gradient (viz. of the order of at least 1000.degree. C.) can be maintained from the inner surfaces of the mold which contact the outer surfaces of the body being compressed to a depth corresponding to a mere "skin" around the body, such as to a depth of about 5 millimeters or less. Consequently, the resulting "skin" which retains its porosity, having been cooled by contact with the mold to a temperature below the pyroplastic temperature range of the body, has a correspondingly small thickness, that is, for example, a thickness of not over about 5 millimeters, such as a thickness of 1 to 5 millimeters.
Since the skin around the compressed body is not only thin, but also relatively smooth and unabraided, due to the procedure for opening the breakaway mold and releasing the body, as described above, in certain applications the compressed bodies, such as ceramic blocks or bricks, can be utilized without removing the skin, or further treatment of the outer surfaces of the bodies. Where desired, however, the skin can be removed by grinding, and, in some cases, grinding may also be desirable to improve the smoothness and dimensional uniformity of the bodies, such as with refractory blocks or bricks, used for constructing furnace walls.